1. Field of the Invention
The present invention relates to a thermally-assisted magnetic recording head including a plasmon generator for use in thermally-assisted magnetic recording where a magnetic recording medium is irradiated with near-field light to lower the coercivity of the magnetic recording medium for data writing.
2. Description of the Related Art
Recently, magnetic recording devices such as magnetic disk drives have been improved in recording density, and thin-film magnetic heads and magnetic recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a read head including a magnetoresistive element (hereinafter, also referred to as MR element) intended for reading and a write head including an induction-type electromagnetic transducer intended for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
Magnetic recording media are discrete media each made of an aggregate of magnetic fine particles, each magnetic fine particle forming a single-domain structure. A single recording bit of a magnetic recording medium is composed of a plurality of magnetic fine particles. For improved recording density, it is necessary to reduce asperities at the borders between adjoining recording bits. To achieve this, the magnetic fine particles must be made smaller. However, making the magnetic fine particles smaller causes the problem that the thermal stability of magnetization of the magnetic fine particles decreases with decreasing volume of the magnetic fine particles. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the magnetic recording medium, and this makes it difficult to perform data writing with existing magnetic heads.
To solve the foregoing problems, there has been proposed a technique so-called thermally-assisted magnetic recording. This technique uses a magnetic recording medium having high coercivity. When writing data, a magnetic field and heat are simultaneously applied to the area of the magnetic recording medium where to write data, so that the area rises in temperature and drops in coercivity for data writing. Hereinafter, a magnetic head for use in thermally-assisted magnetic recording will be referred to as a thermally-assisted magnetic recording head.
In thermally-assisted magnetic recording, near-field light is typically used as a means for applying heat to the magnetic recording medium. A commonly known method for generating near-field light is to use a near-field optical probe or so-called plasmon antenna, which is a piece of metal that generates near-field light from plasmons excited by irradiation with light.
However, the plasmon antenna which generates near-field light by direct irradiation with light is known to exhibit very low efficiency of transformation of the applied light into near-field light. The energy of the light applied to the plasmon antenna is mostly reflected off the surface of the plasmon antenna, or transformed into thermal energy and absorbed by the plasmon antenna. The plasmon antenna is small in volume since the size of the plasmon antenna is set to be smaller than or equal to the wavelength of the light. The plasmon antenna therefore shows a significant increase in temperature when it absorbes the thermal energy.
Such a temperature increase makes the plasmon antenna expand in volume and protrude from a medium facing surface, which is the surface of the thermally-assisted magnetic recording head to face the magnetic recording medium. This causes an end of the read head located in the medium facing surface to get farther from the magnetic recording medium, thereby causing the problem that a servo signal cannot be read during write operations.
There has been known a technique in which a dielectric and a metal are arranged to face each other with a predetermined gap therebetween, and surface plasmons are excited on the metal by utilizing evanescent light that results from the total reflection of the light propagated through the dielectric at the surface of the dielectric. As a related technique, U.S. Pat. No. 7,454,095 discloses a technique in which a metal waveguide and a dielectric waveguide are arranged to face each other with a predetermined gap therebetween, and the metal waveguide is coupled with the dielectric waveguide in a surface plasmon mode. It is then conceivable to establish coupling between the light propagated through the waveguide's core and a plasmon generator, a piece of metal, in a surface plasmon mode through a buffer part so that surface plasmons are excited on the plasmon generator, instead of directly irradiating the plasmon generator with the light. According to such a technique, it is possible to transform the light propagated through the core into near-field light with high efficiency. Since the plasmon generator is not directly irradiated with the light propagated through the core, it is also possible to prevent the plasmon generator from excessively increasing in temperature.
The plasmon generator may be shaped to have an edge part that faces the outer surface of the core with a predetermined distance therebetween. An example of such a shape is a triangular-prism shape. Such a plasmon generator has a front end face that is located in the medium facing surface. The front end face includes a tip that lies at an end of the edge part to form a near-field light generating part. The plasmon generator includes two inclined surfaces that are each connected to the edge part, the two inclined surfaces increasing in distance from each other with increasing distance from the edge part. In the plasmon generator, surface plasmons are excited on the edge part through coupling with the evanescent light that occurs from the outer surface of the core. The surface plasmons are propagated along the edge part to the near-field light generating part located in the medium facing surface, and the near-field light generating part generates near-field light based on the surface plasmons. With such a plasmon generator, it is possible to propagate the surface plasmons excited on the edge part to the near-field light generating part with high efficiency.
In the foregoing plasmon generator, the edge part is ideally formed into a linear shape by the contact of the two inclined surfaces with each other with a predetermined angle formed therebetween. In an actually fabricated plasmon generator, however, the edge part is rounded and thereby has a cylindrical surface configuration that connects the two inclined surfaces forming a predetermined angle therebetween. As employed herein, the radius of curvature of the edge part having the cylindrical surface configuration will be referred to as point radius. The angle that each of the two inclined surfaces forms with respect to the direction perpendicular to the surface of the core that the edge part faces will be referred to as inclination angle. As will be described below, the point radius and the inclination angle of the plasmon generator used in a thermally-assisted magnetic recording head are significant parameters that affect the characteristics of the thermally-assisted magnetic recording head.
First, the point radius will be described. The point radius is a parameter that affects the spot diameter of the near-field light occurring from the near-field light generating part. In order to increase the recording density of a magnetic recording device, a smaller spot diameter is preferred for the near-field light. To reduce the spot diameter of the near-field light, a smaller point radius is preferred.
Next, the inclination angle will be described. To increase the use efficiency of the light propagated through the core of the waveguide, it is important to increase the intensity of the surface plasmons excited on the plasmon generator. This requires that the wave number of the evanescent light and the wave number of the surface plasmons excited on the plasmon generator be matched with each other. The wave number of the surface plasmons excited on the plasmon generator varies according to the shape of the plasmon generator. The inclination angle is thus a parameter that affects the wave number of the surface plasmons excited on the plasmon generator. Meanwhile, the wave number of the evanescent light depends on the wavelength of the light propagated through the core. When typical laser light is used as the light to be propagated through the core, it is necessary that the wave number of the surface plasmons to be excited on the plasmon generator be matched with the wave number of the evanescent light which depends on the wavelength of the laser light. This means that there is a preferred range for the inclination angle.
As seen above, for a plasmon generator having an edge part that faces the core with a buffer part therebetween, the inclination angle needs to fall within the preferred range in order to increase the use efficiency of the light propagated through the core, and the point radius needs to be made smaller in order to make the spot diameter of the near-field light smaller. In order to make the point radius smaller, it is effective to make the inclination angle smaller so that the front end face of the plasmon generator has a tip of more sharply pointed shape. Making the inclination angle smaller, however, gives rise to the problem that the wave number of the surface plasmons to be excited on the plasmon generator does not match with the wave number of the evanescent light. This decreases the surface plasmons to be excited on the edge part, thereby decreasing the use efficiency of the light propagated through the core.
When a thermally-assisted magnetic recording head employs such a configuration that the light propagated through the core is coupled with the plasmon generator in a surface plasmon mode through a buffer part, there arises the following problem if the position of occurrence of the write magnetic field and the position of occurrence of the near-field light are located close to each other. That is, in such a case, both the core and the magnetic pole need to be located near the plasmon generator. It follows that the magnetic pole is located near the core. The magnetic pole is typically made of a magnetic metal material. The presence of such a magnetic pole near the core causes the problem that part of the light propagated through the core is absorbed by the magnetic pole and the use efficiency of the light propagated through the core thereby decreases.